You've probably considered this, but what about the heat treat and alloy of the gears you're considering that have been lightened. Are they a gear that is more intended for long term/mileage use (street, circle track?) and harder material and heat treat, or are you looking at gears that are "softer" to better withstand hard launches of a drag car? I sold a lot of lightened 9" Ford gears for circle track cars but they would have all been the harder "street" heat treat or alloy versions. With lightened gears (ring gear) I think the main advantage would be the reduced inertia to accelerate or decelerate the mass of the gear.
But, I've been a weight fanatic too, but not on my current, budget bracket race FED build. I figure grams add up to ounces which add up to pounds which, before you realize it, your car, airplane or whatever is 50 or 100# fat compared to what it could have been. Some weight is almost impossible to remove once it's built in.
Like I said though, you've probably thought about all of this already.
Lynn